Most of the time, if you put in the comments, something like "used with permission from (copyright owners name), all copyright belongs to (copyright owners name) and cannot be redistributed without their permission." Most people won't bother you but maybe an admin might want to see something in writing. FAQ #193:How do I prove that I had permission to use someone's work? - has a basic memo you can send them for permission. Most of the time, as simple email from the owner saying you can use it, is ok as long as you can show it. Saying "I know this person." or "It's my friend" won't protect you legally. We just don't want you getting into trouble.